29 October 2009

Back in August, I wrote about Yorkshire Forward's decision to drop the RIBA-competition winning footbridge design from Ramboll and Tim Nørlund Jensen on the grounds that its cost had increased beyond the original £1.5m budget, and to replace it with a new design. Mr Jensen was not best pleased, believing he had carried out substantial design development on the original scheme, without ever being paid.

The "iconic" bridge design (shown right), was to span the A630 Sheffield Parkway near Rotherham, providing a new bridleway and footway route.

The new bridge (pictured below) has been designed by Scott Wilson, and is a 49m span hybrid Vierendeel truss and bowstring arch structure.The trusses are 3.5m deep, and made from circular hollow steel sections. It's somewhat similar to a footbridge over the A27 at Tangmere in West Sussex, although not identical: the web members are more highly splayed rather than arranged radially, it lacks the Tangmere bridge's peculiar secondary arch, and the opportunity has been taken at Sheffield to adopt a much more interesting parapet.

So, neither a showpony nor the cheapest substitute, there's a welcome desire to keep it a little more interesting than Yorkshire Forward's budget-consciousness might otherwise have implied.

A number of lessons remain apparent.

RIBA design competitions remain a poor way to get a bridge built, and competitors are accepting an incredibly poor investment-to-reward ratio. Of 393 entries to four open contests (New Islington, Bootle, River Douglas, Sheffield), only 2 made it to planning permission stage (New Islington, Bootle), and only 1 is actually being built (Bootle).

There appears to be little serious scrutiny of the likely costs of designs presented, with Sheffield being one of three recent RIBA contests (the others being River Wear and River Avon) where the winner was found to require far more than the original budget (in two other contests, New Islington and River Douglas, it's unclear if there ever was a real budget).

Promoters simply should not be going down this route if they either lack the money for their bridge, or are unable to sensibly select a design that meets their budget. The cost to the industry is enormous, and it appears that vanity and ambition have often trumped common sense.

28 October 2009

The contest was in two parts: a 9.4m euro opening footbridge spanning 50m across the Inderhavnen (Inner Harbour), won by a design from Flints with Studio Bednarski; and further footbridges spanning Christianshavns Kanal and Trangraven, and across Proviantmagasingraven, won by a design from WTM with Dietmar Feichtinger Architects. There were eight other entrants, with many of the big names in landmark bridge design represented.

The new bridges provide improved walking and cycling links including to the recently completed Opera House. Their construction is being funded privately by the A.P. Møller and Chastine McKinney Møller Foundation, and they are expected to be completed in 2012.

The Inderhavnen structure has already been nicknamed the "kissing bridge" (see right, and below, click on images for larger versions). The two moving halves of the structure retract and extend longitudinally on support rails, like twin brachiosaurs bumping noses when they meet in the middle.

Retractile bridges are very rare, and this bridge will be a nearly unique structure when built. The point of support of the moving span continuously changes as the bridge opens, which normally makes the supporting structure over-heavy compared to a bascule, swing, or lifting bridge, where the reactions are generally carried directly at a bridge pier. Maintenance of the tracks may also be a concern, and the bridge will also be slower to operate than alternatives, as the main spans have to be jacked off their main supports onto the rolling tracks before operation. The jacks, and the interlocking mechanism required at midspan, all add to the likelihood that the bridge will be more expensive to maintain than alternative options. [But see Update below!]

Most retractile bridges are floating structures, where this isn't a problem, and where they can provide larger opening spans than the alternatives (as at the Hood Canal and Ford Island bridges).

In favour of the retractile bridge, the energy costs for operation should be less than a bascule, as the wind loads are much lower. And of course, it will look pretty cool.

The WTM / Feichtinger bridges (see right), which cross smaller arms of the harbour canals, are more conventional single-leaf and double-leaf bascule structures. These have no significant counterweight elements and appear to be supported in both open and close positions on their main hydraulic cylinders, a situation which is far from ideal. Although they're relatively minimalist in appearance, they seem somewhat unconvincing to me.

Several of the other entries are well worth a look, and you can find them online in the jury report at the Copenhagen Kommune website[PDF]. Regretably, I'm a bit too short of time right now to give them a thorough analysis here, but the jury report has plenty of detail on each design. The decision to publish the jury report is something that other competition organisers could learn from: the entrants (all ten of whom were paid a fee for their efforts) receive useful feedback; the scoring process is clearly set out; and the whole exercise is seen to be transparent and rational.

Updated 30 October: Since writing the above, I've been advised that some of the technical details given in the jury report for the Flints design have been changed during the negotiating stage of the contest. In particular, some of the operational aspects have been altered, such that the maintenance costs are likely to be less of an issue than I suggested above. Hopefully more details of the final design will become available in due course.

From the wider perspective, this willingness to fine-tune the design is another example of how sensibly the Copenhagen competition process has been arranged, compared to other cases where a design vision is seen as fixed on day one, despite the limited amount of time a competitor has been given to prepare it.

20 October 2009

I wasn't previously aware of this recent competition to design a new foot and cycle bridge over a highway in Lisbon. It was run over the summer, promoted by Fundação Galp Energia, a Portugese gas and oil distribution company, and ExperimentaDesign, a biennale design exhibition. Open only to registered architects, the scheme proposed a bridge with a 1m euro budget, and the contest had a reasonable prize fund, with 20,000 euros shared amongst the winners.

The organisers received 62 submissions from 14 countries, and the results were announced a couple of weeks ago. I've included whichever images I can find online below. I've left all the entries without comment, so feel free to add comments below.

As always, click on any image for a larger version. Links are provided if the designer's website has more information on a particular design.

09 October 2009

Okay, I got a bit distracted for a few posts there, but it's time to round off my series of posts on low-cost bridges with a few more pointers to organisations involved in building low-cost bridges around the world, or who offer standards and advice on how to design such bridges.

Alpin TechnikThis German contractor sponsored and helped build a timber suspension bridge at Tres Unidos in Peru (pictured below, under construction). Extensive project details are available at the Alpin Technik website, and their presentation at Bridges to Prosperity[PDF] makes interesting reading.ILO/ASISTThe International Labour Organisation runs a programme called ASIST (Advisory Support, Information Services and Training) which (amongst many other things) has published in Zimbabwe a set of Guidelines for the Design and Construction of Suspension Footbridges[PDF]. While this has lots of sound advice, it stops short of including their standard designs.

IT Transport LtdThis UK-based aid consultancy put together Footbridges: A Manual for Construction at Community and District Level on behalf of DfID (UK Department for International Development). This is an excellent and wide-ranging publication, covering bridges made from bamboo and timber as well as simple concrete and steel truss structures. Plenty of references to other sources are made where necessary, and it's based on extensive consultation and real-life case studies.

Overseas Road Note 9This is the UK bible for designing small highway bridges in places that, to us, are far away. Published by the Transport Research Laborator, and titled A design manual for small bridges, it can be found online at DfID's Transport Links website. It's aimed squarely at civil engineers looking for simple bridge designs and details which can be built with partially-skilled labour, and has particularly useful advice on river hydraulics and reinforced concrete bridges.

Global Transport Knowledge PartnershipThe gTKP is funded by DfID, and have developed a set of guidelines for Low Cost Structures for Rural Roads. These overlap to some extent with Overseas Road Note 9, but are generally less technical, and with less focus on bridge structure design. A draft version is available online.

Countryside Commission for ScotlandFor a long time, the CCS's publication Footbridges in the Countryside has been the UK bible for small rural bridges, but it is long out-of-print and not available online, as far as I know. It has been essentially superseded by Paths for All's Path Bridges: Planning, Design, Maintenance and Construction, which is not online but is in print.

I'm conscious this is only a small cross-section of the information available online on low-cost bridges; but hopefully it's a useful starting point for anyone investigating the topic.

07 October 2009

Here's a very interesting (and somewhat controversial) bridge project in Canada, one I spotted recently and felt it deserved more than a one or two-line throwaway in response.

Johnson Street Bridge is a historic heel-trunnion bascule bridge (pictured right, and below left) in the city of Victoria, in British Columbia. Opened in 1924, its two separate parallel decks span 45m and carry both highway and railway (I think the rail span is disused?). It presently carries over 30,000 vehicles every day.

The design, which I guess will be largely unfamiliar to British readers, is to a patent by Joseph Strauss (best known as the designer of the Golden Gate Bridge). As with most bascule bridges, the opening span is balanced by a counterweight suspended behind the main span. It's called a "heel-trunnion" design because the counterweight rotates about a separate trunnion (or axle) to the trunnion used for the bridge deck - this allows the counterweight to be positioned high above the deck, and eliminates the need for a large pit below deck (as is present, for example, on Tower Bridge). In the competing "simple-trunnion" and Scherzer rolling lift designs, the counterweight is rigidly connected to the opening span, and they rotate about a single pivot point.

While the Strauss design is common in North America, the British and European equivalent is a much simpler design, exemplified by the Magere Brug in Amsterdam but also used for larger spans. In both cases, there's a structural mechanism in the form of a parallelogram, which deforms in a diamond shape, maintaining the parallel sides.

In April of this year, the Victoria city authority received a report on the condition of Johnson Street Bridge, which concluded that it was extensively corroded, the mechanical and electrical systems were obsolete and unmaintainable, and the bridge was vulnerable to failure in a large seismic event. The cost of replacement was estimated as competitive against the cost of refurbishment, and Victoria committed to build a new bridge. The consultants MMM and architects Wilkinson Eyre were brought in to draw up options, with a capital budget of CAN$63m in mind.

Conscious of the heritage value of the existing structure, a public consulation committee was set up, and the three design proposals published during September to obtain comments. The options are all unusual and hence interesting, although therefore suffering the disadvantage of a limited proven track record. It's a challenging design: spanning 40m, providing unlimited clearance when open, and it needs to be about 26m wide to carry footways, cycle lanes, highway lanes and the rail corridor. Wide bridges are always difficult to make attractive, and they cause difficulties with opening bridge machinery as well.

All three design options are bascule bridges with raised counterweights. Lifting bridges haven't been considered, presumably because they'd prefer an unlimited air draft, although I reckon a lifting design could be built with less traffic disruption. There's no swing bridge option, presumably because of the site geometry. And there are no conventional bascules, presumably because of the disruption that would be caused building a sunken counterweight pit.

The first design is a "reverse bascule bridge", purportedly inspired by a Vincent van Gogh painting of the European-style heel-trunnion bridge, although it works quite differently. In the Wilkinson Eyre (WEA) design, the counterweight starts horizontally, swinging down to a vertical position. At the same time, a trailing arm on the counterweight, initially vertical, swings up to a horizontal position, pulling up the main span via cables. Essentially, the bridge folds up on itself, and although it's cunningly mirrored and disguised, the principle is essentially the same as on the existing bridge. Instead of a parallelogram, the mechanism flips from a nearly triangular arrangement into a "crossover" quadrilateral.

Without looking at it in more detail, I wonder how well that would really work - bridges of this type are quite sensitive to the position of their centre of gravity, and normally designed to be stable open or closed (i.e. the counterweight tending to maintain the bridge deck in that position). I'm just not sure if that could possibly hold true for this unusual geometry.

Also against this design is its general ungainliness, and the bulk inherent in a 25m wide counterweight - I don't think the renderings really present the width of the structure well, although it's clear that WEA have done all they can to minimise the structure's bulkiness by splitting the footways off onto underslung cantilevers.

The second design is a Calatrava-style cable-stay bridge (i.e. without backstays). I've seen this proposed for bascule bridges, but can't think of any that have been built (any suggestions?) It has the significant disadvantage over the other two options that its centre-of-gravity is somewhere in mid-air, and hence it takes more power to physically push it around.

The best of the three alternatives, and the one chosen by Victoria on 24 September, is a rolling bascule bridge, inspired perhaps by the classic Scherzer rolling lift design, but rotating in place rather than back onto a track, rocking-horse style. A trussed main span sits on two ring girders, each of which presumably sits on bogies or roller supports. It's directly inspired by WEA's similar design at Bellmouth in London's Docklands, a concept also used in Mott MacDonald's Foryd Harbour competition entry, and to an extent in the Falkirk Wheel.

I'm always happy to see a new take on the truss bridge, here replacing the latticework with sleek modern lines and an attractively shaped counterweight. Trusses are rarely the popular choice today, but it would be nice to see them used more often on landmark bridges - they're often the most logical structural form.

Wilkinson Eyre have also put together a pleasing arrangement for the footways, and overall, it's a fine bridge which will undoubtedly be cheaper to maintain in the long-term than the existing structure, with its intricate steelwork and multiple moving parts.

The decision to replace the bridge and adopt a modern design has not been without its opponents. Indeed, there seems to be plenty of dissent, mainly from locals upset at losing a heritage structure and questioning why refurbishment was not a fourth option. They even have their own dedicated website to rival the official project website. It's a great campaigning site - lots of information on the bridge, a regular blog, and opinion which is surprisingly well-informed (as it is elsewhere online, including a helpful Q&A with the lead bridge architect at the Unknown Victoria blog).

Having skimmed the excellent condition report[PDF] on the bridge, I can see both points of view - it's a real shame to lose such an unusual structure, but refurbishment will be very difficult, and will be a cyclic process - the intricate nature of the bridge's truss steelwork is such that it will remain as difficult to maintain in 40 years time as it is now.

It's been alleged by opponents that the decision to replace the bridge and to rush through the design process was largely driven by the opportunity to obtain CAN$42m of federal funding. In an ironic twist, the same day that the council decided on its preferred bridge design, it was announced that the funding would not be forthcoming.

As it stands, Victoria reportedly intends to press ahead, but is taking time out to re-evaluate how best to proceed. This may mean seeking alternative funding, scaling down the replacement bridge plans, or revisiting the refurbishment option. A council meeting tomorrow, 8th October, is scheduled to review the situation and work out where to go next.

As ever, it's hard to tell from photos, but I haven't seen anything to contradict my previous view (see related post below) that it's irrational, exuberant, and spectacular, while making very little sense structurally. The covered walkway seems to detract from the overall appearance, and the tops of the masts seem inelegant and clumsy. Its landmark quality is undeniable, but it will be interesting to see how well it ages. I'd definitely like to visit it to experience it properly!